The present invention relates to a fitting of a member in another, with clearance of several microns or less or even negative clearance, and, particularly, to a method of fitting of two members without gouging and with reduced insertion force.
As an example of known fitting methods, Japanese Patent Application Laid-Open No. 224711/1987 discloses a fitting apparatus such as shown in FIG. 5 of this application. In FIG. 5, a ultrasonic vibration apparatus 41 is constituted with a vibrator 1 and a stepped hone 2 connected to the vibrator 1.
A pin 3 to be inserted into a hole of a part 6 is held in contact with an end of the hone 2 and supported substantially coaxially with the hone 2 by a circular ring 4 provided on a sample holder 5. The ring 4 may be movable horizontally within a limited distance of several millimeters.
The part 6 is supported by a hand portion 7 mounted on an arm 8 of a multi-articulation robot having freedom in a horizontal plane and an altitude and position of the part 6 are controlled by the robot in such a way that the hole of the part 6 is substantially coaxial with the pin 3.
In operation, a fitting of the pin 3 in the hole of the part 6 starts by lowering the part 6 on the robot arm 8. When the pin 3 commences to enter into the hole of the part 6, frictional force is generated therebetween and the pin 3 is urged, by resultant contact reaction, to a top end of the hone 2 on the ultrasonic vibration apparatus 41, by which ultrasonic vibration is transmitted to the pin 3 which just starts to enter into the hole. In this case, the pin 3 is vibrated at a composite frequency of a resonance frequency of the ultrasonic vibrator and a specific frequency of the pin 3. Since amplitude of vibration of the pin 3 is maximum at the top end thereof, frictional force in the contact plane thereof with the part 6 is substantially reduced.
On the other hand, there is a radial vibration in the pin 3 by which the latter expands and contracts radially alternatively. Therefore, in partially fitted state, clearance between the pin and the hole is increased temporarily repeatedly, facilitating the fitting with small insertion force. A model of this operation is illustrated schematically in FIGS. 6a to 6c.
In FIGS. 6a to 6c, a reference numeral 9 depicts a pin and 10 a hole in which the pin 9 is to be fitted. Since the pin 9 vibrates at ultrasonic frequency, it deforms in axial and radial directions alternately as shown by dotted lines in FIG. 6a, in which the pin 9 is shown as being tilted with respect to the hole 10. In FIG. 6a, a top end of the pin 9 contacts with a point A of an inner surface of the hole 10 when the pin 9 is extended axially while being shrinked radially. Due to the axial vibration of the pin 9, friction force at the portion A is substantially reduced.
On the other hand, portion B of an edge of the hole 10 is cleared by the pin 9 when the latter is extended axially while being shrinked radially as shown in FIG. 6a. Thus, top portion of the pin 9 is received in the hole 10.
The inclination of the pin 9 with respect to the hole 10 is corrected by reaction of a contact of a side surface of the pin 9 with the edge of the hole 10 by an axial shrinkage and radial expansion of the pin 9, as shown in FIG. 6b.
Upon a subsequent axial elongation and radial shrinkage of the pin 9, the clearance between the pin 9 and the hole 10 is temporarily enlarged, facilitating the insertion of the pin into the hole.
The above mentioned alternative deformation of the pin 9 is repeated at very high frequency, resulting in a completion of precise fitting of the pin 9 in the hole 10 with relatively small insertion force, as shown in FIG. 6c.
In order to confirm the above mentioned effects caused by the vibration of the pin, insertion force and torsion moment of a pin were detected by mounting the part having a hole through a load sensor mounted on a robot band portion of an apparatus similar to that shown in FIG. 5. FIG. 7a shows a construction of the apparatus used in this experiment. In FIG. 7a, a vibrator 11, a stepped hone 12, a pin 13, a circular ring 14 movable horizontally on a sample support table 15, a part 16 having a hole in which the pin 13 is to be fitted, a robot hand portion 17 and a robot arm 8 are substantially the same as those depicted by reference numerals 1, 2, 3, 4, 6, 7 and 8 in FIG. 5, respectively. A load sensor 17 is provided. FIG. 7b shows a structure of the sensor 17 in detail.
In FIG. 7b, a load cell 19 is provided for measuring of vertical load or insertion force and a cross shaped plate spring 20 is provided for detection of movements around two mutually orthogonal axes which are also orthogonal to a direction of pin insertion. The spring 20 is fixedly secured to an intermedial portion of a support stud 21 which connected to the part 16, and is provided on four arms thereof with strain gauges 22. The load sensor 17 is supported, together with the part 16, by a robot arm 18, with the pin 13 being inserted into the hole of the part by lowering the robot arm 18.
A diameter of the pin 13 was 20 mm and an inner diameter of the hole of the part 16 was selected such that a clearance, i.e., a difference between the outer diameter of the pin and the inner diameter of the hole is 2 .mu.m.
Results of the experiment are shown in FIGS. 8 and 9 for insertion without vibration and for that with vibration, respectively. In these figures, waveforms A and B show moments Mx and My around mutually orthogonal axes orthogonal to a direction of insertion, respectively, which are criteria of catching of the pin by the inner wall of the hole, and waveforms C and D show a vertical load insertion force Fz and a position of the robot arm or insertion amount, respectively. In these figures, abscissa shows time.
In FIG. 8, the moments Mx and My start to increase at a start time point A of lowering of the robot arm, showing deviation of force due to eccentricity. Thereafter, step portions appear on Fz, Mx and My, showing stick slip. When the lowering of the robot arm were continued under these condition, the catching of the pin could occur, making the fitting impossible.
A vibration was applied at a time point 1 to the pin, upon which respective waveform returned to their initial values, the catching problem was solved. When the application of vibration was stopped at a time point 2, the catching occurred again. The latter catching was removed by applying vibration at a time point 3.
In FIG. 9, the same insertion was performed with vibration applied to the pin at time point 2, 4, 6, 8 and 10. In this figure, vibration was stopped at time points 1, 3, 5, 7 and 9. After the vibration was removed at 7, catching appeared immediately which was shown by an abrupt increase of Fz. This was removed by application of vibration at 8.
As is clear from the experiment shown in FIG. 8, in a conventional high precision fitting of a pin in a hole with clearance therebetween in the order of several .mu.m, the insertion becomes impossible due to catching and increased friction. However, by applying vibration to the pin, the problem of catching and increased friction was solved and thus it becomes possible to fit a pin in a hole with minimum insertion force. Further, in FIGS. 8 and 9, it is clear that, by removal of ultrasonic vibration, insertion force and moments are abruptly increased, respectively, which means that frictional force between two members is increased to an extent that fitting cannot be done. This phenomenon can be used to control insertion depth precisely. That is, by measuring insertion depth simultaneously with insertion and by stopping an application of vibration to the pin when measured depth becomes a predetermined value, an insertion operation can be stopped at higher speed than that possible when an operation of robot arm is stopped.
The ultrasonic vibration apparatus 41 used in the assembling robot shown in FIG. 5 is energized immediately before a commencement of insertion operation after the pin 3 is aligned with the hole of the part 6 within a positional error tolerance corresponding to a chamfered portion of the pin 3 and deenergized when the insertion depth becomes equal to a desired value. FIG. 10 is a flowchart showing this operation. Since the ultrasonic vibration apparatus is energized in a short period in an insertion stage, heat generation of the vibrator is minimized, resulting in a elongation of life time of the ultrasonic vibration device.
A principle, an operation and an effect of transmission of ultrasonic vibration to the parts to be fitted in the mentioned fitting apparatus will be described.
Generally, a solid member has a specific resonance frequency determined by its physical condition. When vibration is transmitted to such solid member as the pin 3 through a rigid coupling, it is difficult to obtain a resonation or it is necessary to make the coupling special in configuration. Therefore, it is impossible, in such system, to insert a pin 3 having arbitrary configuration into a hole while applying vibration thereto.
In the above experiment, ultrasonic vibration is coupled to the pin 3 not mechanically but through a ultrasonic vibrator. This is shown in FIG. 11a. The pin 3 to which vibration is transmitted through such ultrasonic vibration is shown in FIG. 11a. Five samples of the pin 3 are prepared whose length l are 50 mm, 70 mm, 100 mm, 170 mm and 220 mm, respectively, and these samples are fitted by a ultrasonic vibrator vibrating at 17.3 kHz. Vibration waveforms of these samples are shown in FIG. 11b. As is clear from FIG. 11b, there is a longitudinal vibration occurred in the pin 3 even when there is no resonance relation between specific frequencies of the ultrasonic vibrator and the pin 3. Therefore, it is enough to transmit such vibration in such a way that its amplitude becomes high enough to produce the effect of reduction of frictional force in a fitting operation.
The assembling fitting by means of the conventional robot is possible with clearance in the order of 10 .mu.m while correcting the inclination of the pin with respect to the hole of the part and, for fitting with clearance in the order of several .mu.m, the application of ultrasonic vibration is effective.
However, when the clearance is small, a fitting becomes very different with the conventional system and, particularly, when the clearance is negative, a fitting must be done by shrink fit or pressure insertion, which requires large load with possibility of damage of fitting surface.